Catalytic two-electron reduction of dioxygen catalysed by metal-free [14]triphyrin(2.1.1)

(1)

Catalytic two-electron reduction of dioxygen

catalysed by metal-free [14]triphyrin(2.1.1)

†

Kentaro Mase,aKei Ohkubo,abZhaoli Xue,cHiroko Yamada*c and Shunichi Fukuzumi*abd

The catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K to yield Me8Fc+and H2O2. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O2 by Me8Fc with HTrip proceedsvia proton-coupled electron transfer from Me8Fc to HTrip to produce H3Tripc+, followed by a second electron transfer from Me8Fc to H3Tripc+to produce H3Trip, which is oxidized by O2via formation of the H3Trip/O2 complex to yield H2O2. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H3Trip to O2in the H3Trip/O2 complex to produce the radical pair (H3Tripc

+

HO2c) as an intermediate, which was detected as a triplet EPR signal with ﬁne-structure by the EPR measurements at low temperature. The distance between the two unpaired electrons in the radical pair was determined to be 4.9˚A from the zero-ﬁeld splitting constant (D).

Introduction

Utilization of natural energy to produce chemical energy con-sisting of earth-abundant elements is an essential technology for building a society based on the sustainable use of materials. Hydrogen peroxide (H2O2) produced by two-electron reduction

of O2is a versatile and environmentally benign oxidant, which

is widely used on a large industrial scale.1,2_{Furthermore, H}

2O2

has been proposed as a sustainable energy carrier that can be used in fuel cells, where direct and eﬃcient conversion of chemical to electrical energy is required.3–5 _{However, the} anthraquinone process, currently used to produce H2O2 in

industry, requires potentially explosive hydrogen and a noble metal catalyst.6 _{Extensive eﬀorts have so far been devoted to} provide an alternative way to produce H2O2photochemically or

thermally without the use of noble metal catalysts.7–13_{In many} cases, redox-active transition metal-based complexes such as cobalt,14–23 _iron,24–27 _{and copper complexes,}28–31 _{have been} employed as O2 reduction catalysts, because triplet O2 is

inactive towards organic compounds due to spin restriction in the absence of an appropriate catalyst.32

Recently, nitrogen-doped carbon materials have attracted increasing attention as an eﬃcient metal-free catalyst for the catalytic reduction of O2.33–35However, the catalytic mechanism

has yet to be well understood, because few spectroscopic studies to detect reaction intermediates in a catalytic cycle have been performed on heterogeneous systems. In homogeneous systems, reduced avin analogues involved in avoenzymes have so far been known to play a pivotal role in the catalytic reduction of O2, which is a key step of biological oxidation.36,37

In particular, the deprotonated states of reduced avin analogues, which are thermodynamically more able to reduce O2 via an electron-transfer process, are considered to be a

reactive intermediate in the reduction of O2.38

On the other hand, Girault and coworkers recently reported that the free base porphyrin has the ability to catalyse the two-electron reduction of O2using one-electron reductants such as

ferrocene at liquid–liquid interfaces.39 _{In such systems,} although the catalytic mechanism of metal-free organocatalysts has yet to be claried, the oxidation state of the organocatalyst is thought to remain the same during the catalytic reduction of O2. Thus, no electron-transfer reduction of organic catalysts has

been reported in relation to the catalytic reduction of O2.

In this context, Nocera and coworkers recently reported the stabilization of the peroxide dianion within the cavity of a hex-acarboxamide cryptand,40_{where strong hydrogen bond donors} are arranged to completely surround the peroxide dianion with a partial positive charge. This result provides support for the proposal that metal-free organocatalysts, which have multiple hydrogen bonding moieties, can eﬃciently catalyse O2reduction. a_{Department of Material and Life Science, Graduate School of Engineering, ALCA and}

SENTAN, Japan Science and Technology Agency (JST), Osaka University, Suita, Osaka 565-0871, Japan. E-mail: [email protected]

b_{Department of Chemistry and Nano Science, Ewha Womans University, Seoul}

120-750, Korea

c_{Graduate School of Materials Science, Nara Institute of Science and Technology,}

CREST, Japan Science and Technology Agency (JST), Ikoma, Nara 630-0192, Japan. E-mail: [email protected]

d_{Faculty of Science and Engineering, ALCA, SENTAN, Japan Science and Technology}

Agency (JST), Meijo University, Nagoya, Aichi 468-0073, Japan

† Electronic supplementary information (ESI) available: Spectroscopic, kinetic and DFT data. See DOI: 10.1039/c5sc02465j

Cite this:Chem. Sci., 2015, 6, 6496

Received 8th July 2015 Accepted 2nd August 2015 DOI: 10.1039/c5sc02465j www.rsc.org/chemicalscience

Science

EDGE ARTICLE

Open Access Article. Published on 03 August 2015. Downloaded on 30/09/2016 07:19:28.

This article is licensed under a

Creative Commons Attribution 3.0 Unported Licence.

View Article Online

(2)

We report herein the catalytic two-electron reduction of O2

by an one-electron reductant, octamethylferrocene (Me8Fc),

with metal-free [14]triphyrin(2.1.1) (denoted as HTrip in Chart 1)41_{in the presence of HClO}

4in benzonitrile (PhCN) at 298 K.

The catalytic mechanism for the O2 reduction by Me8Fc is

claried on the basis of a detailed kinetic study. Proton-coupled electron-transfer reduction of HTrip by Me8Fc results in the

formation of the reduced state of HTrip, and this resulting reduced HTrip is oxidized by O2to reproduce HTrip, indicating

that HTrip acts as a metal-free catalyst for the reduction of O2by

Me8Fc in the presence of HClO4in PhCN. This discovery of a

reactive intermediate in the catalytic O2 reduction with a

molecular organic catalyst provides valuable insight into the development of an eﬃcient metal-free catalyst for the reduction of O2.

Results and discussion

Protonation of HTrip with HClO4

HTrip was protonated by addition of perchloric acid (HClO4) to

an air-saturated benzonitrile (PhCN) solution of HTrip. The characteristic absorption bands for HTrip at 524 and 581 nm decreased in intensity, with an increase in the absorption band at 565 nm, exhibiting clean isosbestic points, as shown in Fig. 1a. As can be seen in Fig. 1b, the absorbance change at 565 nm is saturated in the presence of 1 equiv. of HClO4. Thus,

HTrip is protonated to aﬀord H2Trip+, as given by eqn (1).

HTrip + H+/ H2Trip+ (1)

The pKavalue of H2Trip+in PhCN was estimated to be 15.6 from

the titration of HTrip with triuoroacetic acid (TFA), as shown in Fig. S1 in the ESI.† The pKavalue of H2Trip+is slightly larger

than that of free base porphyrin analogues.42_{There is no further} protonation due to strong repulsion between NH protons in the small macrocyclic ligand, as reported previously.41

Electrochemical measurements of HTrip in the presence of HClO4

Electrochemical measurements of HTrip were performed in deaerated PhCN containing 0.10 M TBAPF6, as shown in Fig. 2.

A cyclic voltammogram of HTrip exhibits reversible reduction waves at E1/2¼ 1.13 and 1.37 V (vs. SCE), which correspond

to therst and second one-electron reduction of HTrip. The rst

one-electron oxidation occurs at E1/2¼ 1.04 V, which is followed

by an irreversible oxidation (Fig. 2a). The formation of HTripc

was detected by UV-vis absorption spectra in the electro-chemical reduction of HTrip at a controlled potential of1.25 V vs. SCE in the thin-layer cell, as shown in Fig. S2 in the ESI.† By addition of HClO4, therst reduction potential of HTrip was

positively shied from E1/2 ¼ 1.13 V to 0.31 V (vs. SCE)

because of the protonation of HTrip, but the reduction became irreversible (Fig. 2b). In such a case, proton-coupled electron transfer from an electron donor with the one-electron oxidation potential, which is less negative than0.31 V, to HTrip may be thermodynamically feasible (vide infra).

Electron-transfer reduction of HTrip in the presence of HClO4

No electron transfer from Me8Fc to HTrip occurred in the

absence of HClO4in PhCN at 298 K, as indicated by the more

negative E1/2value of HTrip (1.13 V vs. SCE) as compared with

that of Me8Fc (0.04 V vs. SCE).8However, the addition of more

than two equiv. of HClO4to a deaerated PhCN solution of Me8Fc

and HTrip resulted in the appearance of an absorption band at 738 nm due to H3Trip with clean isosbestic points, as shown in

Fig. 3. It should be noted that no electron transfer from Me8Fc

to H2Trip+occurred in the presence of one equiv. of HClO4, as

shown in Fig. 3b. These results indicate that uphill electron transfer from Me8Fc to H2Trip+is coupled with protonation of

H2Tripc to produce H3Tripc+, followed by fast electron transfer

from Me8Fc to H3Tripc+ to yield H3Trip. Thus, the second

protonation in fact occurs by coupling with reduction of H2Trip+(i.e. H3Tripc+is accessible but not H3Trip2+). The

stoi-chiometry of the overall reaction is given in Scheme 1. The rate of proton-coupled electron-transfer reduction of H2Trip+ (ket) to form H3Tripc+ was determined from the

dependence of the observed rate constant (kobs) on

concentra-tions of Me8Fc and HClO4, as shown in Fig. 4. The kobsvalue was

determined from the increase in absorbance at 738 nm due to H3Trip, which obeyedrst-order kinetics (Fig. S3 in the ESI†).

The kobsvalue increased linearly with increasing concentrations

of Me8Fc and HClO4, as shown in Fig. 5. Thus, the rate of

formation of H3Trip is given by eqn (2).

d[H3Trip]/dt ¼ ket[H2Trip+][HClO4][Me8Fc] (2) Chart 1 Structure of HTrip.

Fig. 1 (a) Absorption spectral changes of HTrip (2.0 105M) upon the addition of HClO4in air-saturated PhCN at 298 K. (b) Absorbance change proﬁle at 565 nm.

(3)

The ketvalue is determined from the slope of the linear plot of

kobsvs. [Me8Fc] and [HClO4] to be (9.8 0.2) 104M2s1. The

ket value of the proton-coupled electron-transfer reduction of

H2Trip+by Me10Fc was also determined from the slope of the

linear plot of kobsvs. [Me10Fc] and [HClO4] to be (3.1 0.3)

105M2s1(Fig. S4–S6 in the ESI†). The ketvalue for Me10Fc is

larger than that for Me8Fc because Me10Fc (Eox¼ 0.08 V vs.

SCE) is a stronger electron donor than Me8Fc (0.04 V vs.

SCE).28

The formation of H3Trip was also conrmed by the

electro-chemical reduction of H2Trip+monitored by the UV-vis spectral

change at an applied potential of0.30 V vs. SCE in the thin-layer cell, as shown in Fig. S7 (in the ESI†). The product obtained aer the electrochemical reduction of H2Trip+ at

0.30 V displayed the characteristic absorption band at 738 nm. The same absorption band was seen in the chemical reduction of H2Trip+by Me8Fc in the presence of HClO4(Fig. 2).

When O2 was introduced to a deaerated PhCN solution of

H3Trip produced by the proton-coupled electron transfer from

Me8Fc to HTrip in the presence of HClO4, the absorption band

at 738 nm due to H3Trip was immediately changed to a new

absorption band at 720 nm, which can be attributed to the formation of the O2complex, as shown in Scheme 2 (vide infra).

Subsequently, this spectrum decreased gradually, accompanied by the regeneration of HTrip as shown in Fig. 6. This indicates that H3Trip was readily oxidized by O2to produce HTrip and

H2O2(Scheme 2).

Catalytic two-electron reduction of O2by Me8Fc with HTrip in

the presence of HClO4

The proton-coupled electron-transfer reduction of HTrip by Me8Fc (Scheme 1) and the oxidation of the resulting reduced

HTrip (H3Trip) by O2(Scheme 2) indicate that HTrip acts as a

metal-free catalyst for the reduction of O2 by Me8Fc in the

presence of HClO4in PhCN. Indeed, the addition of Me8Fc to

air-saturated PhCN at 298 K containing a catalytic amount of

Fig. 2 Cyclic voltammograms (upper) and diﬀerential pulse voltam-mograms (lower) of deaerated PhCN solutions of HTrip (1.0 103M) recorded in the presence of TBAPF6(0.10 M) (a) without HClO4and (b) with HClO4(1.0 102M); sweep rate: 100 mV s1for CV and 4 mV s1for DPV.

Scheme 1

Fig. 3 (a) Absorption spectral changes upon addition of Me8Fc (1.0 103and 2.0 103M) to a deaerated PhCN solution of H2Trip

+ (1.0 103M) in the presence of HClO4(1.0 103M) at 298 K in a quartz cuvette (light path length¼ 1 mm) (black); absorption spectral change upon addition of HClO4(1.0 103M) to the solution indicated by the black line (red); absorption spectral change upon addition of HClO4 (1.0 103 M) to the solution indicated by the red line (blue). (b) Absorption change at 738 nm upon addition of various concentrations of Me8Fc and HClO4.

Fig. 4 Time proﬁles of absorbance at 738 nm due to H3Trip in the reduction of H2Trip+(2.5 105M) (a) by various concentrations of Me8Fc in the presence of HClO4(3.0 104M) and (b) by Me8Fc (2.0 103_{M) in the presence of various concentrations of HClO}

4 in deaerated PhCN at 298 K.

(4)

HTrip and a large excess of HClO4 resulted in the eﬃcient

oxidation of Me8Fc by O2to yield Me8Fc+, as shown in Fig. 7a.

The formation of Me8Fc+was monitored by a rise in

absor-bance at 750 nm due to Me8Fc+ (Fig. 7b). When an excess

amount of Me8Fc relative to O2(i.e., [O2] limiting conditions)

was employed, the concentration of produced Me8Fc+ (1.9

103M) was twice that of O2(9.4 104M). In addition, the

stoichiometric production of H2O2 was conrmed by

iodo-metric titration, as shown in Fig. S8 (in the ESI†). In contrast, when an excess amount of O2 relative to Me8Fc (i.e., [Me8Fc]

limiting conditions) was employed, the concentration of produced H2O2(1.0 103 M) was half that of Me8Fc (2.0

103 M), where the amount of H2O2 was determined by the

reaction with [(TMC)FeII](OTf)2 (TMC ¼

1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) to produce the correspond-ing Fe(IV)-oxo complex ([TMC]FeIV(O))2+, as shown in Fig. S9 (in the ESI†).43_{Thus, the stoichiometry of the catalytic reduction of} O2by Me8Fc has beenrmly established, as given in eqn (3).

O2þ 2Hþþ 2Me8Fc!

HTrip H2O2þ 2Me8Fc

þ ₍₃₎

The rate of formation of Me8Fc+in the catalytic reduction of

O2 with excess Me8Fc and HClO4 in Fig. 7b obeys rst-order

kinetics. It should be noted that the oxidation of Me8Fc by O2

hardly occurred in the absence of HTrip under the present experimental conditions, as shown in Fig. S10 (in the ESI†). When Me8Fc was replaced by weaker one-electron reductants

such as ferrocene (Fc : Eox ¼ 0.37 V vs. SCE) and

dime-thylferrocene (Me2Fc : Eox¼ 0.26 V vs. SCE), no changes in the

absorption band of H2Trip+at 565 nm were observed, as shown

in Fig. S11 (in the ESI†). When Me8Fc was replaced by a stronger

one-electron reductant, i.e., decamethylferrocene (Me10Fc : Eox

¼ 0.10 V vs. SCE), greatly enhanced oxidation of Me10Fc

occurred with the decrease in absorbance at 565 nm due to H2Trip+(Fig. S12a in the ESI†). In the case of Me10Fc, however,

the oxidation of Me10Fc by O2 occurred without HTrip in the

presence of HClO4in PhCN (Fig. S12c in the ESI†). These results

indicate that the reduction of H2Trip+ to produce H3Trip is

essential in the catalytic reduction of O2to produce H2O2.

When a metal complex of HTrip,h5-cyclopentadienyliron(II)

[14]triphyrin(2.1.1) (CpFeIITrip),41c _{was employed as an O}

2

reduction catalyst instead of HTrip for comparison, however, the addition of HClO4 to an air-saturated PhCN solution of

CpFeIITrip resulted in a spectral change, as shown in Fig. S13 (in the ESI†). The characteristic absorption bands of CpFeII_Trip

at 545 nm and 608 nm disappeared upon the addition of HClO4

with the appearance of new absorption bands at 565 nm, which can be attributed to those of H2Trip+. This indicates that

CpFeII_{Trip was easily demetallated and protonated to aﬀord}

H2Trip+in the presence of HClO4, as shown in Fig. S13 (in the

ESI†).

Fig. 5 (a) Plot ofkobsvs. [Me8Fc] for the reduction of H2Trip +

(2.5 105M) by various concentrations of Me8Fc in the presence of HClO4 (3.0 104M) in PhCN at 298 K. (b) Plot ofkobsvs. [HClO4] for the reduction of H2Trip+(2.5 105M) by Me8Fc (2.0 103M) in the presence of various concentrations of HClO4in deaerated PhCN at 298 K.

Scheme 2

Fig. 6 (a) Absorption spectral changes produced by electron transfer from Me8Fc (1.0 104M) to HTrip (2.5 105M) in the presence of HClO4 (1.0 104 M) in deaerated PhCN at 298 K. (b) Absorption spectral changes upon introducing O2to a deaerated PhCN solution of (a). The red and green lines show the spectrum of H3Trip before and after introducing O2by O2gas bubbling, respectively. The blue line shows the spectrum due to precursor complex. Insets show absorp-tion time proﬁles.

Fig. 7 (a) Absorption spectral changes in the two-electron reduction of O2(9.4 104M) by Me8Fc (1.0 102M) with HTrip (5.0 105M) in the presence of HClO4(1.0 102M) in PhCN at 298 K. The black and red lines show the spectra before and after addition of Me8Fc, respectively. The dotted line is the absorbance at 750 nm due to 1.9 103M of Me8Fc+. (b) Time proﬁle of absorbance at 750 nm due to Me8Fc+. Inset showsﬁrst-order plot.

(5)

Kinetics and mechanism of the catalytic two-electron reduction of O2by Me8Fc with HTrip

The dependence of therst-order rate constant for the forma-tion of Me8Fc+on the concentrations of HTrip, HClO4, Me8Fc,

and O2was examined, as shown in Fig. S14 (in the ESI†), where

therst-order rate constants were determined from the initial slopes of therst-order plots in order to avoid further compli-cation due to the deactivation of the catalyst during the reac-tions, as shown in Fig. S15 (in the ESI†). The observed rst-order rate constant (kobs) was proportional to the concentration of

HTrip, whereas the kobsvalue remained constant irrespective of

the concentration of HClO4 or Me8Fc (Fig. 8). Although no

degradation of HTrip occurred under the present acidic condi-tions (Fig. S16 in the ESI†), the turnover number (TON) based on HTrip was determined to be more than 40 when the lower concentration of HTrip (1.3 105M) was employed, as shown in Fig. S14a (in the ESI†). Because the catalytic rate depends only on the concentrations of HTrip and O2, the rate-determining

step in the catalytic cycle must be the reaction of H3Trip with O2

in Scheme 3. The dependence of the initial rate of formation of Me8Fc+on the concentration of O2shows saturation behaviour

at large concentrations of O2(Fig. 8d). Such saturation

behav-iour is consistent with the formation of the O2complex (H3Trip/

O2) in the oxidation of H3Trip with O2(Fig. 6b and Scheme 3).

The overall catalytic cycle is shown in Scheme 3, where proton-coupled electron transfer from Me8Fc to HTrip is followed by a

second electron transfer from Me8Fc to H3Tripc+ to produce

H3Trip, which is slowly oxidized by O2via the H3Trip/O2complex

as the rate-determining step. Because the direct reaction between H3Trip and O2 in the H3Trip/O2 complex is

spin-forbidden, the reaction may proceed via hydrogen atom transfer from H3Trip to O2 in the H3Trip/O2 complex to produce the

(H2Tripc/HO2c) intermediate, followed by a rapid second

hydrogen transfer from H2Tripc to HO2c to yield H2O2,

accom-panied by regeneration of HTrip (Scheme 3). According to Scheme 3, the rate of formation of Me8Fc+is given by eqn (4),

d[Me8Fc+]/dt ¼ kcat[H3Trip/O2], (4)

where kcatis the rate constant of the hydrogen atom transfer

from H3Trip to O2 in the H3Trip/O2 complex. Because the

concentration of the H3Trip/O2complex is given by eqn (5)

[H3Trip/O2] ¼ K[HTrip][O2]/(1 + K[O2]), (5)

using the formation constant (K), the initial concentration of HTrip, which is converted to H3Trip in the catalytic reaction,

and the concentration of O2, eqn (4) is rewritten as eqn (6).

d[Me8Fc+]/dt ¼ kcatK[HTrip][O2]/(1 + K[O2]) (6)

This kinetic equation agrees with the experimental observations in Fig. 8. The kcat and K values were determined from the

dependence of the catalytic rate on the concentration of O2

(Fig. 8d) to be 0.5 s1and 8.4 102M1, respectively.

Although the radical pair (H2Tripc/HO2c) in Scheme 3 cannot

be detected during the catalytic reaction, the formation of the

radical pair (H2Tripc/HO2c) was successfully detected by EPR

measurements using 1-benzyl-1,4-dihydronicotinamide dimer [(BNA)2]44as an electron donor to produce H3Trip under

pho-toirradiation at low temperature. The observed EPR spectrum in aerated PhCN in the presence of HClO4at low temperature is

shown in Fig. 9. A tripletne structure EPR signal was observed as well as the typical anisotropic signals for HO2c with the g||

value of 2.0341, and isotropic signals for H2Tripc at 2.0030.45,46

From the zero-eld splitting value (D ¼ 230 G), the distance (r) between two unpaired electrons was determined using the relation D¼ 27 800/r347_{to be 4.9 ˚}_{A. This distance is consistent} with the estimated distance between O2 and H3Trip in the

H3Trip/O2complex by DFT calculations (Fig. 9b). Scheme 3

Fig. 8 Plots of (a)kobsvs. [HTrip] for the two-electron reduction of O2 (9.4 104M) by Me2Fc (1.0 102M) with various concentrations of HTrip in the presence of HClO4(1.0 102M) in PhCN; (b)kobsvs. [HClO4] for the two-electron reduction of O2(9.4 104M) by Me8Fc (1.0 102M) with HTrip (5.0 105M) in PhCN at 298 K; (c)kobsvs. [Me8Fc] for the two-electron reduction of O2(9.4 104M) by various concentrations of Me8Fc with HTrip (5.0 105M) in the presence of HClO4(1.0 102M) in PhCN at 298 K; and (d)kobsvs. [O2] for the two-electron reduction of O2by Me8Fc (1.0 102M) with HTrip (5.0 105_{M) in the presence of HClO}

4(1.0 102M) in PhCN at 298 K.

(6)

Conclusion

Metal-free triphyrin acts as an eﬃcient catalyst for the two-electron reduction of O2 by Me8Fc to produce H2O2 in the

presence of HClO4in PhCN at 298 K. The rate-determining step

(RDS) in the catalytic cycle has been found to be hydrogen atom transfer from H3Tip to O2in the H3Trip/O2complex to produce

the radical pair (H3Tripc+/HO2c), which was detected as a triplet

species by EPR at 80 K. The distance between the two unpaired electrons (4.9 ˚A) determined from the zero-eld splitting constant (D) agrees with the distance in the H3Trip/O2complex

calculated by DFT. The present study provides valuable insight into the catalytic mechanism of the two-electron reduction of O2

with an organic catalyst, and may lead to the development of more eﬃcient metal-free organic catalysts for the selective two-electron reduction of O2to produce H2O2.

Experimental section

General procedure

Chemicals were purchased from commercial sources and used without further purication, unless otherwise noted. Perchloric acid (HClO4, 70%), triuoroacetic acid (TFA), ferrocene (Fc), and

1,1-dimethylferrocene (Me2Fc) were purchased from Wako Pure

Chemical Industries Ltd. Octamethylferrocene (Me8Fc) and

decamethylferrocene (Me10Fc) were received from Sigma

Aldrich. Fc, Me2Fc, Me8Fc, and Me10Fc were puried by

subli-mation or recrystallization from ethanol. Benzonitrile (PhCN) used for spectroscopic and electrochemical measurements was distilled over phosphorus pentoxide prior to use.48 [14]Tri-phyrin(2.1.1) [HTrip] was synthesized according to the reported procedure.41 _Fe(_II_)(TMC)(OTf)

2 (TMC ¼

1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf¼ CF3SO3) was prepared

according to a literature method.43 _{Tetra-n-butylammonium} hexauorophosphate (TBAPF6) was twice recrystallized from

ethanol and dried in vacuo prior to use.1H NMR spectra (300 MHz) were recorded on a JEOL AL-300 spectrometer at room temperature and chemical shis (ppm) were determined rela-tive to tetramethylsilane (TMS). UV-vis absorption spectroscopy was carried out on a Hewlett Packard 8453 diode array spec-trophotometer at room temperature using a quartz cell (light path length¼ 1 cm).

Spectroscopic measurements

The amount of hydrogen peroxide (H2O2) produced was

deter-mined by titration with iodide ion: a dilute CH3CN solution

(2.0 mL) of the product mixture (50mL) was treated with an excess amount of NaI, and the amount of I3 formed was

determined from the absorption spectrum (lmax¼ 361 nm, 3 ¼

2.8 104M1cm1).49_{The formation of H}

2O2in the catalytic O2

reduction with HTrip was again conrmed by the reaction between H2O2and Fe(II)(TMC)(OTf)2to aﬀord the

correspond-ing Fe(IV)-oxo species. The amount of the Fe(IV)-oxo

species produced was determined from the absorption spec-trum (lmax¼ 820 nm, 3 ¼ 400 M1cm1).43

The turnover numbers (TON¼ the number of moles of H2O2

formed per mole of HTrip in the catalytic two-electron reduction of O2) were determined from the concentration of produced

Me8Fc+ under catalytic conditions, where stoichiometric

production of H2O2was conrmed by iodometric titration.

Kinetic measurements

Rate constants of oxidation of ferrocene derivatives by O2in the

presence of a catalytic amount of HTrip and an excess amount of HClO4in PhCN at 298 K were determined by monitoring the

appearance of an absorption band due to the corresponding ferrocenium ions (Fc+,lmax¼ 620 nm, 3max¼ 330 M1cm1;

Me2Fc+,lmax¼ 650 nm, 3max¼ 290 M1cm1; Me8Fc+,lmax¼

750 nm,3max¼ 410 M1cm1; Me10Fc+,lmax¼ 780 nm, 3max¼

450 M1cm1).14_{At the wavelengths monitored, spectral} over-lap was observed with H3Trip (l ¼ 738 nm (3 ¼ 1.6 103M1

cm1)), H3Trip/O2(l ¼ 720 nm (3 ¼ 1.2 103M1cm1)). The

concentration of O2 in an air-saturated PhCN solution was

determined to be 1.7 103M as reported previously.50 _The concentrations of ferrocene derivatives employed for the cata-lytic reduction of O2were much larger than that of O2, as O2is

the rate-limiting reagent in the reaction solution. The PhCN solutions containing various concentrations of O2 for the

kinetic measurements were prepared by N2/O2 mixed gas

bubbling using a KOFLOC GASBLENDER GB-3C. Typically, a PhCN stock solution of a ferrocene derivative was added using a

Fig. 9 EPR spectrum observed after the reduction of HTrip (1.0 103 M) by (BNA)2(2.0 103M) in the presence of HClO4(1.0 103M) in aerated PhCN under photoirradiation using a high-pressure Hg lamp (1000 W) measured at 80 K. Experimental conditions: Microwave frequency 9.0 GHz, microwave power 1.0 mW, modulation frequency 100 kHz, and modulation width 10 G. (b) Optimized structure of H3Trip/O2calculated by DFT with calculated spin-density values given in parentheses at the UB3LYP/6-31G(d) level of theory.

(7)

microsyringe to a PhCN solution containing HTrip and HClO4

in a quartz cuvette (light path length¼ 1 cm). Electrochemical measurements

Cyclic voltammetry (CV) measurements were performed on an ALS 630B electrochemical analyser and voltammograms were measured in deaerated PhCN containing 0.10 M TBAPF6as a

supporting electrolyte at room temperature. A conventional three-electrode cell was used with a glassy carbon working electrode (surface area of 0.3 mm2) and a platinum wire as the counter electrode. The glassy carbon working electrode (BAS) was routinely polished with BAS polishing alumina suspension and rinsed with acetone before use. The potentials were measured with respect to the Ag/AgNO3(1.0 102M)

refer-ence electrode. All potentials (vs. Ag/AgNO3) were converted to

values vs. SCE by adding 0.29 V.51_{Redox potentials were} deter-mined using the relation E1/2¼ (Epa+ Epc)/2.

Spectroelectrochemical measurements

UV-visible spectroelectrochemical experiments were performed with a home-built thin-layer cell (1 mm) that had a light transparent platinum net working electrode. Potentials were applied and monitored with an ALS 730D electrochemical analyser.

EPR measurements

EPR spectra were measured on a JEOL X-band EPR spectrometer (JES-ME-LX) using a quartz EPR tube containing a deaerated frozen sample solution at 80 K. The internal diameter of the EPR tube is 4.0 mm, which is small enough toll the EPR cavity but large enough to obtain good signal-to-noise ratios during the EPR measurements at low temperatures (at 80 K). EPR spectrum of HTripc_{produced by the electrochemical reduction}

of HTrip was measured using a home-built three-electrode quartz EPR tube. Potentials were applied and monitored with an ALS 730D electrochemical analyser. EPR spectra were measured under nonsaturating microwave power conditions. The ampli-tude of modulation was chosen to optimize the resolution and the signal-to-noise (S/N) ratio of the observed spectra. The g values were calibrated with a Mn2+marker.

Theoretical calculations

Density functional theory (DFT) calculations were performed on a 32CPU workstation (PQS, Quantum Cube QS8-2400C-064). Geometry optimisations were carried out using the B3LYP/ 6-31G(d) level of theory52 for HTripc_{, H}

2Trip+, H3Trip2+,

H3Tripc+, and [H3Trip/O2]. All calculations were performed

using Gaussian 09, revision A.02.53 _{Graphical outputs of the} computational results were generated with the GaussView so-ware program (ver. 3.09) developed by Semichem, Inc.54

Acknowledgements

This work was supported by grants from the Advanced Low Carbon Technology Research and Development (ALCA) and

SENTAN programs from Japan Science Technology Agency (JST) to S. F., the Japan Society for the Promotion of Science (JSPS: Grants 26620154 and 26288037 to K. O., 25288092 and 26105004 to H. Y. and 25-727 to K. M.), and the Green Photonics Project in NAIST sponsored by the MEXT (Japan).

Notes and references

1 (a) S. Abrantes, E. Amaral, A. P. Costa, A. A. Shatalov and A. P. Duarte, Ind. Crops Prod., 2007, 25, 288–293; (b) S. H. Zeronian and M. K. Inglesby, Cellulose, 1995, 2, 265– 272.

2 L. Li, S. Lee, H. L. Lee and H. J. Youn, BioResources, 2011, 6, 721–736.

3 (a) S. Yamazaki, Z. Siroma, H. Senoh, T. Ioroi, N. Fujiwara and K. Yasuda, J. Power Sources, 2008, 178, 20–25; (b) R. S. Disselkamp, Energy Fuels, 2008, 22, 2771–2774; (c) R. S. Disselkamp, Int. J. Hydrogen Energy, 2010, 35, 1049– 1053.

4 S. Fukuzumi, Y. Yamada and K. D. Karlin, Electrochim. Acta, 2012, 82, 493–511.

5 (a) Y. Yamada and S. Fukuzumi, Aust. J. Chem., 2014, 67, 354– 364; (b) Y. Yamada, M. Yoneda and S. Fukuzumi, Inorg. Chem., 2014, 53, 1272–1274; (c) Y. Yamada, M. Yoneda and S. Fukuzumi, Chem.–Eur. J., 2013, 19, 11733–11741; (d) Y. Yamada, S. Yoshida, T. Honda and S. Fukuzumi, Energy Environ. Sci., 2011, 4, 2822–2825; (e) Y. Yamada, Y. Fukunishi, S. Yamazaki and S. Fukuzumi, Chem. Commun., 2010, 46, 7334–7336.

6 E. Santacesaria, M. D. Serio, R. Velotti and U. Leone, Ind. Eng. Chem. Res., 1994, 33, 277–281.

7 M. Fukushima, K. Tatsumi, S. Tanaka and H. Nakamura, Environ. Sci. Technol., 1998, 32, 3948–3953.

8 S. Liu, K. Mase, C. Bougher, S. D. Hicks, M. M. Abu-Omar and S. Fukuzumi, Inorg. Chem., 2014, 53, 7780–7788.

9 A. J. Hoﬀman, E. R. Carraway and M. R. Hoﬀmann, Environ. Sci. Technol., 1994, 28, 776–785.

10 (a) V. Maurino, C. Minero, G. Mariella and E. Pelizzetti, Chem. Commun., 2005, 2627–2629; (b) M. Teranishi, S. Naya and H. Tada, J. Am. Chem. Soc., 2010, 132, 7850– 7851; (c) D. Tsukamoto, A. Shiro, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka and T. Hirai, ACS Catal., 2012, 2, 599–603.

11 S. Kato, J. Jung, T. Suenobu and S. Fukuzumi, Energy Environ. Sci., 2013, 6, 3756–3764.

12 (a) Y. Yamada, A. Nomura, K. Ohkubo, T. Suenobu and S. Fukuzumi, Chem. Commun., 2013, 49, 5132–5134; (b) Y. Yamada, A. Nomura, T. Miyahigashi and S. Fukuzumi, Chem. Commun., 2012, 48, 8329–8331.

13 (a) S. Fukuzumi and K. Ohkubo, Chem. Sci., 2013, 4, 561–574; (b) S. Fukuzumi and K. Ohkubo, Org. Biomol. Chem., 2014, 12, 6059–6071; (c) K. Ohkubo and S. Fukuzumi, Bull. Chem. Soc. Jpn., 2009, 82, 303–315; (d) H. Kotani, K. Ohkubo and S. Fukuzumi, Appl. Catal., B, 2008, 77, 317–324; (e)

K. Ohkubo, R. Iwata, K. Mizushima, K. Souma,

Y. Yamamoto, N. Suzuki and S. Fukuzumi, Chem. Commun., 2010, 46, 601–603.

(8)

14 (a) K. Mase, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 2800–2808; (b) T. Honda, T. Kojima and S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 4196–4206; (c) S. Fukuzumi, Chem. Lett., 2008, 37, 808–813; (d) S. Fukuzumi, K. Okamoto, C. P. Gros and R. Guilard, J. Am. Chem. Soc., 2004, 126, 10441–10449; (e) S. Fukuzumi, K. Okamoto, Y. Tokuda, C. P. Gros and R. Guilard, J. Am. Chem. Soc., 2004, 126, 17059–17066.

15 (a) S. Fukuzumi, S. Mochizuki and T. Tanaka, Inorg. Chem., 1989, 28, 2459–2465; (b) S. Fukuzumi, S. Mochizuki and T. Tanaka, Inorg. Chem., 1990, 29, 653–659; (c) S. Fukuzumi, S. Mochizuki and T. Tanaka, J. Chem. Soc., Chem. Commun., 1989, 391–392.

16 (a) R. McGuire Jr, D. K. Dogutan, T. S. Teets, J. Suntivich, Y. Shao-Horn and D. G. Nocera, Chem. Sci., 2010, 1, 411– 414; (b) D. K. Dogutan, S. A. Stoian, R. McGuire Jr, M. Schwalbe, T. S. Teets and D. G. Nocera, J. Am. Chem. Soc., 2011, 133, 131–140; (c) T. S. Teets, T. R. Cook, B. D. McCarthy and D. G. Nocera, J. Am. Chem. Soc., 2011, 133, 8114–8117.

17 (a) F. C. Anson, C. Shi and B. Steiger, Acc. Chem. Res., 1997, 30, 437–449; (b) C. Shi, B. Steiger, M. Yuasa and F. C. Anson, Inorg. Chem., 1997, 36, 4294–4295; (c) C. Shi and F. C. Anson, Inorg. Chem., 1998, 37, 1037–1043; (d) Z. Liu and F. C. Anson, Inorg. Chem., 2000, 39, 274. 18 (a) A. Schechter, M. Stanevsky, A. Mahammed and Z. Gross,

Inorg. Chem., 2012, 51, 22–24; (b) J. Masa, K. Ozoemena,

W. Schuhmann and J. H. Zagal, J. Porphyrins

Phthalocyanines, 2012, 16, 761–784.

19 (a) A. J. Olaya, D. Schaming, P.-F. Brevet, H. Nagatani, T. Zimmermann, J. Vanicek, H.-J. Xu, C. P. Gros, J.-M. Barbe and H. H. Girault, J. Am. Chem. Soc., 2012, 134, 498–506; (b) P. Peljo, L. Murtom¨aki, T. Kallio, H.-J. Xu, M. Meyer, C. P. Gros, J.-M. Barbe, H. H. Girault, K. Laasonen and K. Kontturi, J. Am. Chem. Soc., 2012, 134, 5974–5984; (c) B. Su, I. Hatay, A. Troj´anek, Z. Samec, T. Khoury, C. P. Gros, J.-M. Barbe, A. Daina, P.-A. Carrupt and H. H. Girault, J. Am. Chem. Soc., 2010, 132, 2655–2662. 20 (a) S. Fukuzumi, S. Mandal, K. Mase, K. Ohkubo, H. Park,

J. Benet-Buchholz, W. Nam and A. Llobet, J. Am. Chem. Soc., 2012, 134, 9906–9909; (b) T. Wada, H. Maki, T. Imamoto, H. Yuki and Y. Miyazato, Chem. Commun., 2013, 49, 4394–4396.

21 (a) K. M. Kadish, L. Fremond, J. Shen, P. Chen, K. Ohkubo, S. Fukuzumi, M. El Ojaimi, C. P. Gros, J.-M. Barbe and R. Guilard, Inorg. Chem., 2009, 48, 2571–2582; (b) P. Chen, H. Lau, B. Habermeyer, C. P. Gros, J.-M. Barbe and K. M. Kadish, J. Porphyrins Phthalocyanines, 2011, 15, 467– 479; (c) K. M. Kadish, L. Fremond, Z. Ou, J. Shao, C. Shi, F. C. Anson, F. Burdet, C. P. Gros, J.-M. Barbe and R. Guilard, J. Am. Chem. Soc., 2005, 127, 5625–5631. 22 (a) C. J. Chang, Y. Deng, D. G. Nocera, C. Shi, F. C. Anson and

C. K. Chang, Chem. Commun., 2000, 1355–1356; (b) C. J. Chang, Z. H. Loh, C. Shi, F. C. Anson and D. G. Nocera, J. Am. Chem. Soc., 2004, 126, 10013–10020. 23 (a) E. Askarizadeh, S. B. Yaghoob, D. M. Boghaei,

A. M. Slawin and J. B. Love, Chem. Commun., 2010, 46,

710–712; (b) M. Volpe, H. Hartnett, J. W. Leeland, K. Wills, M. Ogunshun, B. J. Duncombe, C. Wilson, A. J. Blake, J. McMaster and J. B. Love, Inorg. Chem., 2009, 48, 5195– 5207.

24 (a) R. A. Decr´eau, J. P. Collman and A. Hosseini, Chem. Soc. Rev., 2010, 39, 1291–1301; (b) J. P. Collman, R. Boulatov, C. J. Sunderland and L. Fu, Chem. Rev., 2004, 104, 561–588. 25 (a) J. Rosenthal and D. G. Nocera, Acc. Chem. Res., 2007, 40, 543–553; (b) M. Schwalbe, D. K. Dogutan, S. A. Stoian, T. S. Teets and D. G. Nocera, Inorg. Chem., 2011, 50, 1368– 1377; (c) C. J. Chang, L. L. Chng and D. G. Nocera, J. Am. Chem. Soc., 2003, 125, 1866–1876.

26 (a) Z. Halime, H. Kotani, Y. Li, S. Fukuzumi and K. D. Karlin, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 13990–13994; (b) E. E. Chufan, S. C. Puiu and K. D. Karlin, Acc. Chem. Res., 2007, 40, 563–572; (c) E. Kim, E. E. Chufan, K. Kamaraj and K. D. Karlin, Chem. Rev., 2004, 104, 1077–1133. 27 (a) C. T. Carver, B. D. Matson and J. M. Mayer, J. Am. Chem.

Soc., 2012, 134, 5444–5447; (b) B. D. Matson, C. T. Carver, A. von Ruden, J. Y. Yang, S. Raugei and J. M. Mayer, Chem. Commun., 2012, 48, 11100–11102; (c) J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010, 110, 6961– 7001.

28 (a) S. Kakuda, R. L. Peterson, K. Ohkubo, K. D. Karlin and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 6513–6522; (b) D. Das, Y.-M. Lee, K. Ohkubo, W. Nam, K. D. Karlin and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 2825–2834; (c) D. Das, Y.-M. Lee, K. Ohkubo, W. Nam, K. D. Karlin and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 4018–4026; (d) S. Fukuzumi, L. Tahsini, Y.-M. Lee, K. Ohkubo, W. Nam and K. D. Karlin, J. Am. Chem. Soc., 2012, 134, 7025–7035. 29 (a) S. Fukuzumi and K. D. Karlin, Coord. Chem. Rev., 2013,

257, 187–195; (b) L. Tahsini, H. Kotani, Y.-M. Lee, J. Cho, W. Nam, K. D. Karlin and S. Fukuzumi, Chem.–Eur. J., 2012, 18, 1084–1093; (c) S. Fukuzumi, H. Kotani, H. R. Lucas, K. Doi, T. Suenobu, R. L. Peterson and K. D. Karlin, J. Am. Chem. Soc., 2010, 132, 6874–6875. 30 (a) J. Zhang and F. C. Anson, J. Electroanal. Chem., 1993, 348,

81–97; (b) Y. Lei and F. C. Anson, Inorg. Chem., 1994, 33, 5003–5009; (c) Y. C. Weng, F.-R. F. Fan and A. J. Bard, J. Am. Chem. Soc., 2005, 127, 17576–17577.

31 (a) M. S. Thorum, J. Yadav and A. A. Gewirth, Angew. Chem., Int. Ed., 2009, 48, 165–167; (b) C. C. L. McCrory, A. Devadoss, X. Ottenwaelder, R. D. Lowe, T. D. P. Stack and C. E. D. Chidsey, J. Am. Chem. Soc., 2011, 133, 3696–3699; (c) M. A. Thorseth, C. E. Tornow, E. C. M. Tse and A. A. Gewirth, Coord. Chem. Rev., 2013, 257, 130–139; (d) M. A. Thorseth, C. S. Letko, T. B. Rauchfuss and A. A. Gewirth, Inorg. Chem., 2011, 50, 6158–6161.

32 C. Kemal, T. W. Chan and T. C. Bruice, J. Am. Chem. Soc., 1977, 99, 7272–7286.

33 (a) K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760–764; (b) S. Wang, D. Yu and L. Dai, J. Am. Chem. Soc., 2011, 133, 5182–5185.

34 (a) Q. Li, B. W. Noe, Y. Wang, B. Menezes, D. G. Peters, K. Raghavachari and L.-S. Li, J. Am. Chem. Soc., 2014, 136,

(9)

3358–3361; (b) Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, J. Am. Chem. Soc., 2014, 136, 4394–4403.

35 L. Wang, A. Ambrosi and M. Pumera, Angew. Chem., Int. Ed., 2013, 52, 13818–13821.

36 (a) S. Shibata, T. Suenobu and S. Fukuzumi, Angew. Chem., Int. Ed., 2013, 52, 12327–12331; (b) S. Fukuzumi, K. Yasui, T. Suenobu, K. Ohkubo, M. Fujitsuka and O. Ito, J. Phys. Chem. A, 2001, 105, 10501–10510; (c) S. Fukuzumi, S. Kuroda, T. Goto, K. Ishikawa and T. Tanaka, J. Chem. Soc., Perkin Trans. 2, 1989, 1047–1053.

37 (a) S. Ghisla and V. Massey, Eur. J. Biochem., 1989, 181, 1–17; (b) T. C. Bruice, Acc. Chem. Res., 1980, 13, 256–262; (c) V. Massey, J. Biol. Chem., 1994, 269, 22459–22462.

38 S. Fukuzumi and T. Okamoto, J. Chem. Soc., Chem. Commun., 1994, 521–522.

39 (a) I. Hatay, B. Su, M. A. Méndez, C. Corminboeuf, T. Khoury, C. P. Gros, M. Bourdillon, M. Meyer, J.-M. Barbe, M. Ersoz, S. Záliˇs, Z. Samec and H. H. Girault, J. Am. Chem. Soc., 2010, 132, 13733–13741; (b) A. Trojánek, J. Langmaier and Z. Samec, Electrochim. Acta, 2012, 82, 457–462; (c) A. Trojánek, J. Langmaier, B. Su, H. H. Girault and Z. Samec, Electrochem. Commun., 2009, 11, 1940–1943; (d) S. Wu and B. Su, Chem.–Eur. J., 2012, 18, 3169–3173; (e) X. Liu, S. Wu and B. Su, J. Electroanal. Chem., 2013, 709, 26. 40 N. Lopez, D. J. Graham, R. McGuire Jr, G. E. Alliger, Y. Shao-Horn, C. C. Cummins and D. G. Nocera, Science, 2012, 335, 450–453.

41 (a) Z.-L. Xue, J. Mack, H. Lu, L. Zhang, X.-Z. You, D. Kuzuhara, M. Stillman, H. Yamada, S. Yamauchi, N. Kobayashi and Z. Shen, Chem.–Eur. J., 2011, 17, 4396– 4407; (b) Z.-L. Xue, Z. Shen, J. Mack, D. Kuzuhara,

H. Yamada, T. Okujima, N. Ono, X.-Z. You and

N. Kobayashi, J. Am. Chem. Soc., 2008, 130, 16478–16479; (c) Z.-L. Xue, D. Kuzuhara, S. Ikeda, Y. Sakakibara, K. Ohkubo, N. Aratani, T. Okujima, H. Uno, S. Fukuzumi and H. Yamada, Angew. Chem., Int. Ed., 2013, 52, 7306–7309. 42 I. Kaljurand, A. Kütt, L. Sooväli, T. Rodima, V. Mäemets, I. Leito and I. A. Koppel, J. Org. Chem., 2005, 70, 1019–1028. 43 (a) J.-U. Rohde, J.-H. In, M. H. Lim, W. W. Brennessel, M. R. Bukowski, A. Stubna, E. Münck, W. Nam and L. Que Jr, Science, 2003, 299, 1037–1039; (b) E. S. Yang, M.-S. Chan and A. C. Wahl, J. Phys. Chem., 1975, 79, 2049–2051. 44 S. Fukuzumi, T. Suenobu, M. Patz, T. Hirasaka, S. Itoh,

M. Fujitsuka and O. Ito, J. Am. Chem. Soc., 1998, 120, 8060– 8068.

45 (a) K. Suga, K. Ohkubo and S. Fukuzumi, J. Phys. Chem. A, 2005, 109, 10168–10175; (b) H. Kotani, K. Ohkubo and

S. Fukuzumi, J. Am. Chem. Soc., 2004, 126, 15999–16006; (c) S. Fukuzumi and K. Ohkubo, Chem.–Eur. J., 2000, 6, 4532– 4535.

46 (a) R. N. Bagchi, A. M. Bond, F. Scholz and R. St¨osser, J. Am. Chem. Soc., 1989, 111, 8270–8271; (b) M. Bersohn and J. R. Thomas, J. Am. Chem. Soc., 1964, 86, 959–959; (c) J. A. Howard, in Peroxyl radicals, ed. Z. B. Alfassi, Wiley, Chichester, 1997, p. 283.

47 (a) J. S. Park, E. Karnas, K. Ohkubo, P. Chen, K. M. Kadish, S. Fukuzumi, C. W. Bielawski, T. W. Hudnall, V. M. Lynch and J. L. Sessler, Science, 2010, 329, 1324–1326; (b) S. Fukuzumi, K. Ohkubo, Y. Kawashima, D. S. Kim, J. S. Park, A. Jana, V. Lynch, D. Kim and J. L. Sessler, J. Am. Chem. Soc., 2011, 133, 15938–15941.

48 W. L. F. Armarego and C. L. L. Chai, Purication of Laboratory Chemicals, Pergamon Press, Oxford, 7th edn, 2013.

49 S. Fukuzumi, S. Kuroda and T. Tanaka, J. Am. Chem. Soc., 1985, 107, 3020–3027.

50 (a) S. Fukuzumi, H. Imahori, H. Yamada, M. E. El-Khouly, M. Fujitsuka, O. Ito and D. M. Guldi, J. Am. Chem. Soc., 2001, 123, 2571–2575; (b) S. Fukuzumi, M. Ishikawa and T. Tanaka, J. Chem. Soc., Perkin Trans. 2, 1989, 1037–1045. 51 C. K. Mann and K. K. Barnes, in Electrochemical Reactions in

Non-Aqueous Systems, Mercel Dekker, New York, 1970. 52 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.

53 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, revision A.02, Gaussian, Inc., Wallingford, CT, 2009.

54 R. Dennington II, T. Keith, J. Millam, K. Eppinnett, W. L. Hovell and R. Gilliland, Gaussview, Semichem, Inc., Shawnee Mission, KS, 2003.